Semiconductor processing often involves the deposition of films or layers over or on a semiconductor substrate surface which may or may not have other layers already formed thereon. In typical circuitry fabrication, portions of an outer layer are masked, typically using photoresist, to provide a desired pattern over the outer layer. An underlying layer is then etched through the mask opening, with the mask covering and protecting the substantial portions of the covered portions from etching. Typically it is desired to etch the outer layer or layers selectively relative to an underlying layer. Accordingly, etch chemistry and conditions are continually being developed and improved to achieve a manner by which the desired layer can be etched while stopping and substantially not etching an underlying layer.
One common material in semiconductor processing which is desirably etched selectively relative to certain underlying material is doped and undoped polysilicon. In the context of this document, "doped polysilicon" defines a polysilicon material which is doped with a conductivity enhancing impurity to a concentration of at least 1.times.10.sup.18 atoms/cm.sup.3. Accordingly, "undoped polysilicon" in the context of this document defines a polysilicon material having less than 1.times.10.sup.18 atoms/cm.sup.3 of conductivity enhancing dopant impurity. Typical materials that polysilicon is desirably selectively etched relative to include deposited oxides, whether doped or undoped, thermally grown oxides, and nitrides, for example silicon nitride.
One etch chemistry which is useful and effective at etching polysilicon is referred to as "APE", which is also known as an ammonia peroxide etch chemistry, which comprises a combination of ammonium fluoride, hydrogen peroxide, and acetic acid. A typical prior art composition of such an aqueous solution includes a mixture having a volumetric ratio of such materials at 250:250:2.5, respectively. The NH.sub.4 F is however provided as a 45% aqueous solution by volume, the H.sub.2 O.sub.2 as a 70% aqueous solution by volume, with the acetic acid being substantially 100% pure acetic acid. Accordingly, the true ratio of these components by volume in the mixture of such solutions is 112.5:175:2.5, by volume. This etching chemistry is quite effective at etching deposited polysilicon layers. However, its selectivity to deposited oxides such as borophosphosilicate glass (BPSG), is almost non-existent, meaning that such chemistry etches these materials at substantially the same rate. Accordingly, it would be desirable to develop a method for etching polysilicon substantially selective relative to deposited oxides, thermal oxides and nitrides utilizing a chemistry that includes NH.sub.4 F, at least one oxidizer, and acetic acid.